How to prevent sculpting tools from affecting faces through thin walls?
Try enabling Front Faces only:

If that doesn't work, then try painting the vertices you don't want to change with the mask brush:

These masked vertices won't be allowed to change. To clear the mask, press ⎇ AltM.
UPDATE: In version 2.8 or higher of Blender, the Front Faces Only option can be found in the tool's header bar, within the Brush menu.